Lasers scanners employing hologon discs have been developed for a variety of applications including bar code scanning in electronic point of sale systems and various types of imaging systems. U.S. Pat. No. 4,610,500 issued to Kramer on Sept. 9, 1986, for example, discloses a hologon laser scanner apparatus that includes a hologon scanner disc and a motor for rotating the hologon scanner disc. The hologon scanner disc is coupled to the motor with a shaft that is inserted into a hub provided in the center of the hologon disc. The end of the shaft defines a shoulder that bears against the lower surface of the hub. A screw is used to clamp the hub against the shoulder. The hologon scanner disc includes one or more deflection gratings which are used to deflect the path of a laser beam focused on the disc as the disc is rotated by the motor. A lens is provided to focus the deflected laser beam on a image surface.
Problems have been experienced with laser scanners of the type disclosed in U.S. Pat. No. 4,610,500, namely those having a disc coupled to a driving motor by a shaft. As described in Japanese Kokai No. 59-101068, the hologon scanner disc is susceptible to the formation of tiny cracks due to vibrations at high speeds where the disc is mounted to the motor drive shaft. Japanese Kokai No. 59-101068 proposes to eliminate the problems associated with the mounting of hologon scanner discs on drive shafts by making the scanner disc part of the rotor of the motor. Specifically, the scanner disc is mounted in the inner circumference of a ring that has a number of magnets located on its outer circumference. The ring is mounted in a frame structure made of a magnetic material. A field-generating magnet is mounted within the frame structure. The overall structure constitutes a linear motor with the frame structure acting as the primary core and the magnets located on the ring as the secondary conductor. Application of an electrical signal to the field-generating magnets causes the ring, with the scanner disc included, to rotate.
The use of the hologon scanner disc as part of the motor rotor provides advantages over mounting the disc to a drive shaft in addition to the elimination of the cracks described above, for example, the overall size of the scanning apparatus is greatly reduced. At the same time, however, additional problems are created which are particularly detrimental to printing systems employing the scanning apparatus. The hologon scanner disc has a tendency to heat up in a nonuniform manner due to the heat generated in the stator and rotor windings. The nonuniform heating causes mechanical strain and/or shifting of the grating provided on the disc, which in turn causes the laser beam to be improperly deflected. Thus, the optical distortions of the disc due to nonuniform heating directly result in degradation of image quality.
In view of the above, it is an object of the invention to provide a holographic scanner motor (a motor that utilizes the hologon scanning disc as part of the rotor) having an improved structure that aids in maintaining the temperature stability of the hologon scanner disc to prevent optical distortions due to nonuniform heating.
It is a further object of the invention to provide a holographic scanner motor having an improved structure which aids in the cooling of hologon scanner disc that is compact in design.